In the prior art, as shown in FIG. 7, the diversity receiving device that is equipped with a plurality of receiving systems and executes the diversity reception has been proposed (see Patent Literature 1, for example). The conventional diversity receiving device shown in FIG. 7 is constructed to include two antennas 101, 102 used as diversity antennas, a tuner portion 103 for turning in to a desired channel from a digital modulation signal caught by the antenna 101, a tuner portion 104 for turning in to a desired channel from a digital modulation signal caught by the antenna 102, a demodulating portion 105 for demodulating a signal on the channel chosen by the tuner portion 103, a demodulating portion 106 for demodulating a signal on the channel chosen by the tuner portion 104, a synthesizing portion 107 for synthesizing the signal demodulated by the demodulating portion 105 and the signal demodulated by the demodulating portion 106, a received field strength sensing portion 108 for sensing a received field strength based on the signal demodulated by the demodulating portion 105 and the signal demodulated by the demodulating portion 106, a fading pitch sensing portion 109 for sensing a fading pitch based on the received field strength sensed by the received field strength sensing portion 108, a synthesis-stop deciding portion 110 for deciding a stop of a signal synthesis in the synthesizing portion 107 based on the fading pitch sensed by the fading pitch sensing portion 109 and the received field strength sensed by the received field strength sensing portion 108, and a power-supply controlling portion 111 for ON/OF-controlling a power supply of the block whose power supply should be shut down when the diversity operation is stopped.
Here, the antenna 101, the tuner portion 103, and the demodulating portion 105 and the antenna 102, tuner portion 104, the demodulating portion 106 constitute one receiving system respectively. The former receiving system is called a first branch 1200, and the latter receiving system is called a second branch 1300.
Next, an operation of the diversity receiving device constructed as above will be explained hereunder. The tuner portion 103 turns in to the desired channel from the digital modulation signal caught by the antenna 101. At the same time, the tuner portion 103 also turns in to the desired channel from the digital modulation signal caught by the antenna 102. Then, the signal on the channel chosen by the tuner portion 103 is demodulated by the demodulating portion 105, and the signal on the channel chosen by the tuner portion 104 is demodulated by the demodulating portion 106. Then, signals demodulated by the demodulating portions 105 and 106 respectively are synthesized by the synthesizing portion 107, and a synthesized signal is output as demodulated data. Also, the signals demodulated by the demodulating portions 105 and 106 respectively are input into the received field strength sensing portion 108, and the received field strength is calculated. The field strength value calculated by the received field strength sensing portion 108 is input into the fading pitch sensing portion 109, and the fading pitch is sensed.
Then, the fading pitch and the field strength value are input into the synthesis-stop deciding portion 110, a threshold best suited to stop the synthesis is calculated based on a value of the fading pitch. Then, when the field strength value satisfies the threshold, a command to stop a synthesizing operation is sent to the synthesizing portion 107 from the synthesis-stop deciding portion 110. Also, a command to stop a supply of power to the unnecessary branch is sent to the power-supply controlling portion 111 from the synthesis-stop deciding portion 110. For example, when a command to stop a supply of power to the first branch 1200 is issued, a supply of power to the first branch 1200 is stopped. Accordingly, only the demodulated signal from the second branch 1300 is input into the synthesizing portion 107. Then, the demodulated signal from the second branch 1300 is output as demodulated data. In this manner, the diversity receiving device in the prior art implements a lower power consumption in such a way that the fading pitch is sensed, then the diversity stop conditions that are most suitable in the present fading circumstance is decided, and either the synthesis is stopped or the supply of power to the unnecessary branch is stopped.    Patent Literature 1: JP-A-2000-357983